Expanded Graphite and Process for Producing the Expanded Graphite

ABSTRACT

An expanded graphite is derived from a graphitic or partially graphitic starting material selected from the group consisting of natural graphite, compressed expanded graphite, partially oxidized graphite and/or graphite fibers having a BET surface area of &gt;30 m 2 /g. The expanded graphite is obtained by reaction of the starting material with substances capable of intercalation or mixtures of substances capable of intercalation to give a compound designated as an intercalation compound and subsequent expansion in plasma.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the priority, under 35 U.S.C. § 119, of Germanapplication DE 10 2007 053 652.8, filed Nov. 8, 2007; the priorapplication is herewith incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION Field of the Invention

The invention relates to an expanded graphite produced from a graphiticor partially graphitic starting material selected from the groupconsisting of natural graphite, compressed expanded graphite, partiallyoxidized graphite and graphite fibers, with the starting material beingreacted with substances capable of intercalation or mixtures ofsubstances capable of intercalation to give a compound referred tohereinafter as intercalation compound and subsequently being expanded.The invention further relates to the use of these materials.

German patent DE 66804 C discloses the production of expanded graphiteparticles having a worm-like structure by thermal decomposition of agraphite intercalation compound as is obtained, for example, by actionof concentrated sulfuric acid or a mixture of nitric acid and sulfuricacid on natural graphite particles. The expanded particles, hereinafterreferred to as expanded graphite, are extraordinarily malleable and havehigh specific surface areas. The shapeability of the expanded graphiteparticles, the strength and flexibility of the articles produced fromthe particles and the specific surface area of the particles aredetermined essentially by the degree of expansion, which for the presentpurposes is the expansion of the particles in the direction of the caxis of the graphite crystal, which can be quantified by an increase involume of a particle bed. According to German patent DE 1253130 C1,corresponding to U.S. Pat. Nos. 3,494,382 and 3,404,061, the degree ofexpansion should be at least 80, preferably at least 200, since shapedbodies produced by pressing together of expanded particles without theaddition of a binder then acquire sufficient strength.

The expansion of the graphite particles is obviously attributable to thefact that during heating expanding gaseous decomposition and/orvaporization products of the intercalation compounds push the layers orpackets of layers of the graphite crystal apart. This process ends withthe breakout of the gas which is initially enclosed within theindividual grains and the degree of expansion is approximately inverselyproportional to the amount of gas escaping during the heating phase (M.B. Dowell, 12. Conf. on Carbon. Jul. 28 to Aug. 1, 1975, Pittsburgh,Pa., p. 31). To produce graphite particles which have a high degree ofexpansion and a good processability and a high specific surface area, itis necessary for significantly more gas to be produced in the interiorof the solid than can flow out through resulting channels, cracks andpores for a certain period of time.

SUMMARY OF THE INVENTION

It is accordingly an object of the invention to provide an expandedgraphite and process for producing the expanded graphite that overcomethe above-mentioned disadvantages of the prior art methods and devicesof this general type. With the foregoing and other objects in view thereis provided, in accordance with the invention, a method for producing anexpanded graphite. The method includes selecting a starting materialfrom natural graphite, compressed expanded graphite, partially oxidizedgraphite, and/or graphite fibers having a BET surface area of >30 m²/g;reacting the starting material with substances capable of intercalationor mixtures of substances capable of intercalation resulting in acompound designated as an intercalation compound; and performing asubsequent expansion in plasma.

The object is achieved by treating the starting material of the typementioned at the outset with a plasma.

It has surprisingly been found that plasma can, under the chosenconditions, achieve such a high heating rate of the material thatexpansion occurs despite a very short residence time of theintercalation compound in the hot plasma zone, which can be in themillisecond range. Expanded graphites having a BET surface area of >30m²/g are obtained.

Furthermore, it has been found that an expanded graphite obtained inthis way has a surface area which is greater than that of the expandedgraphite known from the prior art and can be chemically andmorphologically modified. This enables a disadvantage of the knownexpanded graphites, namely a low wettability of the material for manymedia, to be overcome. The novel expanded graphites can, for example, beused as an absorbent material for liquid or gaseous media or asaggregate for composite materials.

Compared to the processes known from the prior art for producingexpanded graphites, the process of the invention allows a significantlyhigher process versatility in respect of the production of chemicallyand morphologically modified expanded graphites. The expansion and themodification can be brought about in only one step. The processtherefore has process engineering advantages over the sequential processdescribed in published, non-prosecuted German patent application DE 102007 023 315 A1 titled “Process for Producing a Latent Heat StorageMaterial”. For example, the specific surface area and the surfacemorphology of the expanded material can be modified by introduction ofoxygen or other gases having a dry etching action as a process gas.Furthermore, functional chemical surface groups can be produced on thesurfaces of the expanded material which are in contact with the gasphase by introduction of functionalizing process gases.

The object of simultaneous expansion and modification is achieved byexposing the material to be expanded to the action of plasma to whichone or more process gases can be added. For this purpose, the materialto be expanded can be moved as a bed on a transport device through theplasma, sprinkled as individual particles through the plasma, sprayed orheld in a fluidized bed in the plasma zone. The plasma is preferablyproduced as a localized zone, for example as a plasma flame, laser focusor as extended excited region, for example a microwave discharge.

In this procedure, the plasma serves as a source of high-energy gaseousspecies, for example rotationally, vibrationally and/or electronicallyexcited molecules or free radicals, electronically excited atoms or ionsof the surrounding gas atmosphere and also electrons and photons. Thesespecies transfer sufficient enthalpy from the plasma gas phase to theintercalation compound for the heating rate and residence time of thematerial introduced to be sufficient to expand the intercalationcompound. In addition, the chemically active constituents of the gasatmosphere can act on the chemical bonds of the surface of theintercalation compound or of the expanded graphite which is in contactwith the gas phase in such a way that bond ruptures in the surface andsubsequently formation of reaction products with species of the gasphase occur. These reactions make themselves apparent in the form offunctional surface groups or lead to ablation of material. The type ofchemical or morphological modification of the particle surface takingplace simultaneously with expansion as a result of the plasma can beinfluenced by choice of the plasma process gases added and/or by thetype of intercalation compound. As a result, the plasma can have anetching, chemically modifying or coating action. Many differentfunctional groups or closed or open layers can in this way be producedon expanded surfaces. These include oxygen-containing,nitrogen-containing, halogen-containing, silicon-containing,phosphorus-containing, metal-containing and other groups or layerscomposed of these.

Expanded graphites which have been surface-functionalized in this wayhave improved wettability for selected liquid or gaseous media. They canbe used, for example, as adsorbent materials. Functional groups orlayers can also help improve the dispersibility of expanded graphites,which is helpful for the production of composite materials containinghomogeneously distributed graphite particles. A specificfunctionalization or coating can additionally lead to a chemicalinteraction between expanded graphite and the surrounding matrix of acomposite material, by which, for example, the mechanical or thermalconduction or electrical properties of the composite can be influencedin a positive way.

The energy necessary for production and operation of the plasma can beintroduced into the process gas by use of ions, electrons, electric orelectromagnetic fields including radiation. Industrially, excitation ofa gas plasma suitable for expansion of intercalation compounds can beachieved in a very high pressure range, preferably in the high pressurerange from 50,000 to 150,000 Pa, particularly preferably in the regionof atmospheric pressure, by use of a DC gas discharge or high-frequencyor low-frequency AC gas discharge, a high-energy electromagneticradiation field, as is produced, for example, by a microwave source or alaser, or by a source of electrons or ions.

When a laser is used in the process described here, this must, incontrast to the expansion by laser radiation described in Europeanpatent application EP 87489 A1, exceed a critical radiation densityabove which a plasma which additionally heats the intercalation compoundby the radiation in the form of photons and also via the enthalpy inputof rotationally, vibrationally and/or electronically excited gasmolecules, atoms, ions or free radicals is formed in the laser focus.

In the process of the invention, the plasma is operated discontinuouslyor, preferably, continuously. The temperature of the uncharged gascomponent of the plasma should preferably be above 500 K.

The plasma can be generated in process gases under reduced atmosphericpressure and also under superatmospheric pressure, and for the processto be carried out simply, it is preferably generated in process gasesunder or close to atmospheric pressure.

The expansion according to the invention of the intercalation compoundsin the plasma allows very high expansion rates at very short processtimes which can be in the millisecond range. The degree of expansion canbe controlled within a wide range via the plasma power and the residencetime of the particles in the hot plasma zone.

The size of the particles of an intercalation compound which can beexpanded by plasma extends from a number of millimeters down into thetwo-decimal-place nanometer range. Apart from intercalated graphitic orpartially graphitic compounds, it is also possible for intercalatedgraphitic carbon fibers and intercalated graphitic carbon nanofibers tobe expanded by means of the process.

Other features which are considered as characteristic for the inventionare set forth in the appended claims.

Although the invention is described herein as embodied in an expandedgraphite and process for producing the expanded graphite, it isnevertheless not intended to be limited to the details described, sincevarious modifications and structural changes may be made therein withoutdeparting from the spirit of the invention and within the scope andrange of equivalents of the claims.

The construction and method of operation of the invention, however,together with additional objects and advantages thereof will be bestunderstood from the following description of specific embodiments.

DETAILED DESCRIPTION OF THE INVENTION

The invention is illustrated below with the aid of examples.

Comparative Example 1

Commercially available graphite hydrogen sulfate SS3 (from SumikinChemical Co., Ltd., Tokyo, Japan) was heated in a shock-like fashion to1,000° C. in a muffle furnace. The expanded material obtained in thisway had a density of 5 kg m³. Its chemical composition was, according toXPS analysis, C=97.1 atom % (at. %); O=1.9 at. %. The expanded graphiteobtained has a BET surface area of 19 m²/g. The water absorption of ashaped body having a density of 500 kg/m³ composed of the pressedexpanded material was, after storage in distilled water for fiveminutes, less than 10% of the mass of the shaped body.

Example 1

Commercially available graphite hydrogen sulfate SS3 was blown atatmospheric pressure into an inductively coupled thermal plasma flameexcited at 4 MHz. 70 standard liters per minute (slm) of argon wereadded as the process gas. The electric power introduced was 1.45 kW. Thefeed rate of the graphite hydrogen sulfate was 7.3 g/60 s and its inflowvelocity was 5.9 m/s. The expanded material obtained had a density of4.7 kg/m³. Its chemical composition was, according to XPS analysis, C=95at. % and O=5 at. %. The energy efficiency was 3.3 kWh/kg of expandedmaterial. The expanded graphite obtained has a BET surface area of 38m²/g. The water absorption of a shaped body having a density of 500kg/m³ composed of the pressed expanded material was, after storage indistilled water for five minutes, less than 303% of the mass of theshaped body.

Example 2

Commercially available graphite hydrogen sulfate SS3 was blown atatmospheric pressure into an inductively coupled thermal plasma flameexcited at 4 MHz. 120 slm of argon were added as a process gas. Theelectric power introduced was 9.6 kW. The feed rate of the graphitehydrogen sulfate was 3.1 g/60 s and its inflow velocity was 1.5 m/s. Theexpanded material obtained had a density of 1.9 kg/m³. Its chemicalcomposition was, according to XPS analysis, C=97.7 at. % and O=2.3 at.%. The energy efficiency was 51.9 kWh/kg of expanded material. Theexpanded graphite obtained has a BET surface area of 45 m²/g. The waterabsorption of a shaped body having a density of 500 kg/m³ composed ofthe pressed expanded material was, after storage in distilled water forfive minutes, less than 41% of the mass of the shaped body.

Example 3

Commercially available graphite hydrogen sulfate SS3 was blown atatmospheric pressure into an inductively coupled thermal plasma flameexcited at 4 MHz. 120 slm of argon were added as process gas. Theelectric power introduced was 5.4 kW. The feed rate of the graphitehydrogen sulfate was 3.0 g/60 s and its inflow velocity was 5.9 m/s. Theexpanded material obtained had a density of 4.1 kg/m³. Its chemicalcomposition was, according to XPS analysis, C=96.9 at. % and O=3.1 at.%. The energy efficiency was 27.2 kWh/kg of expanded material. Theexpanded graphite obtained has a BET surface area of 42 m²/g. The waterabsorption of a pellet composed of the pressed expanded material was,after storage in distilled water for five minutes, less than 270% of themass of the pellet.

Example 4

Commercially available graphite hydrogen sulfate SS3 was blown atatmospheric pressure into an inductively coupled thermal plasma flameexcited at 4 MHz. 120 slm of argon were added as the process gas. Theelectric power introduced was 5.8 kW. The feed rate of the graphitehydrogen sulfate was 3.0 g/min and its inflow velocity was 53.1 m/s. Theexpanded material obtained had a density of 15.2 kg/m³. Its chemicalcomposition was, according to XPS analysis, C=96.1 at. % and O=3.9 at.%. The energy efficiency was 32.2 kWh/kg of expanded material. Theexpanded graphite obtained has a BET surface area of 30 m²/g. The waterabsorption of a shaped body having a density of 500 kg/m³ composed ofthe pressed expanded material was, after storage in distilled water forfive minutes, less than 439% of the mass of the shaped body.

1. A method for producing an expanded graphite, which comprises thesteps of: selecting a starting material from the group consisting ofnatural graphite, compressed expanded graphite, partially oxidizedgraphite, and graphite fibers having a BET surface area of >30 m²/g;reacting the starting material with one of substances capable ofintercalation and mixtures of substances capable of intercalationresulting in a compound designated as an intercalation compound; andperforming a subsequent expansion in plasma.
 2. The method according toclaim 1, which further comprises performing a heating step which leadsto expansion of the intercalation compound as a result of enthalpy inputof a process gas excited by means of the plasma.
 3. The method accordingto claim 1, which further comprises treating the intercalation compoundin the plasma of an electrostatic field.
 4. The method according toclaim 1, which further comprises treating the intercalation compound inthe plasma of at least one electromagnetic AC field.
 5. The methodaccording to claim 4, which further comprises treating the intercalationcompound in the plasma having an electromagnetic excitation frequency isbelow 100 Hz.
 6. The method according to claim 4, which furthercomprises treating the intercalation compound in the plasma having anelectromagnetic excitation frequency is in a low-frequency range from100 Hz to 10 kHz.
 7. The method according to claim 4, which furthercomprises treating the intercalation compound in the plasma having anelectromagnetic excitation frequency is in a radiofrequency range from10 kHz to 300 MHz.
 8. The method according to claim 4, which furthercomprises treating the intercalation compound in the plasma having anelectromagnetic excitation frequency is in a microwave range from 300MHz to 300 GHz.
 9. The method according to claim 4, which furthercomprises treating the intercalation compound in the plasma having anelectromagnetic excitation frequency is in a range above 300 GHz. 10.The method according to claim 1, which further comprises treating theintercalation compound in the plasma having activating process gasesselected from the group consisting of the noble gases.
 11. The methodaccording to claim 1, which further comprises treating the intercalationcompound in the plasma to which oxidizing process gases such as air,oxygen, carbon dioxide, water or solutions containing hydrogen peroxideare added.
 12. The method according to claim 1, which further comprisestreating the intercalation compound in the plasma to which reducingprocess gases such as hydrogen are added.
 13. The expanded graphiteaccording to claim 1, which further comprises treating the intercalationcompound in the plasma to which process gases selected from the groupconsisting of gases which generate nitrogen-containing,halogen-containing, silicon-containing, phosphorus-containing andsulfur-containing functional groups, are added.
 14. The expandedgraphite according to claim 10, which further comprises treating theintercalation compound in the plasma to which at least one process gasis added.
 15. The method according to claim 5, which further comprisessetting the electromagnetic excitation frequency at a grid frequency of50 or 60 Hz.
 16. The method according to claim 7, which furthercomprises setting the electromagnetic excitation frequency at a multipleof the industrially supplied 13.56 MHz.
 17. The method according toclaim 8, which further comprises setting the electromagnetic excitationfrequency at a multiple of the industrially supplied 2.45 GHz.
 18. Themethod according to claim 9, which further comprises treating theintercalation compound with laser radiation.
 19. An expanded graphite,comprising: a starting material selected from the group consisting ofnatural graphite, compressed expanded graphite, partially oxidizedgraphite, and graphite fibers having a BET surface area of >30 m²/g; andone of substances capable of intercalation and mixtures of substancescapable of intercalation reacting with said starting material resultingin a compound designated as an intercalation compound and a subsequentexpansion in plasma.
 20. The expanded graphite according to claim 19,wherein heat is provided for expansion of said intercalation compound asa result of an enthalpy input of a process gas excited by means of theplasma.
 21. The expanded graphite according to claim 19, wherein saidintercalation compound is treated in the plasma of an electrostaticfield.
 22. The expanded graphite according to claim 19, wherein saidintercalation compound is treated in the plasma of at least oneelectromagnetic AC field.
 23. The expanded graphite according to claim22, wherein said intercalation compound is treated in the plasma havingan electromagnetic excitation frequency below 100 Hz.
 24. The expandedgraphite according to claim 22, wherein said intercalation compound istreated in the plasma having an electromagnetic excitation frequency ina low-frequency range from 100 Hz to 10 kHz.
 25. The expanded graphiteaccording to claim 22, wherein said intercalation compound is treated inthe plasma having an electromagnetic excitation frequency in aradiofrequency range from 10 kHz to 300 MHz.
 26. The expanded graphiteaccording to claim 22, wherein said intercalation compound is treated inthe plasma having an electromagnetic excitation frequency in a microwaverange from 300 MHz to 300 GHz.
 27. The expanded graphite according toclaim 22, wherein said intercalation compound is treated in the plasmahaving an electromagnetic excitation frequency in a range above 300 GHz.28. The expanded graphite according to claim 19, wherein saidintercalation compound is treated in the plasma having activatingprocess gases selected from the group consisting of the noble gases. 29.The expanded graphite according to claim 19, wherein said intercalationcompound is treated in the plasma to which oxidizing process gasesselected from the group consisting of air, oxygen, carbon dioxide, waterand solutions containing hydrogen peroxide, are added.
 30. The expandedgraphite according to claim 19, wherein said intercalation compound istreated in the plasma to which reducing process gases are added.
 31. Theexpanded graphite according to claim 19, wherein said intercalationcompound is treated in the plasma to which process gases selected fromthe group consisting of gases which generate nitrogen-containingfunctional groups, halogen-containing functional groups,silicon-containing functional groups, phosphorus-containing functionalgroups and sulfur-containing functional groups are added.
 32. Theexpanded graphite according to claim 28, wherein said intercalationcompound is treated in the plasma to which at least one process gas isadded.
 33. The expanded graphite according to claim 23, wherein theelectromagnetic excitation frequency is at a grid frequency of 50 or 60Hz.
 34. The expanded graphite according to claim 25, wherein theelectromagnetic excitation frequency is set at a multiple of theindustrially supplied 13.56 MHz.
 35. The expanded graphite according toclaim 26, wherein the electromagnetic excitation frequency is set at amultiple of the industrially supplied 2.45 GHz.
 36. The expandedgraphite according to claim 27, wherein said intercalation compound istreated with laser radiation.
 37. The expanded graphite according toclaim 30, wherein said reducing process gases include hydrogen.
 38. Amethod of using a graphite, which comprises the steps of: providing anexpanded graphite formed from a starting material selected from thegroup consisting of natural graphite, compressed expanded graphite,partially oxidized graphite, and graphite fibers having a BET surfacearea of >30 m²/g and one of substances capable of intercalation andmixtures of substances capable of intercalation reacting with thestarting material resulting in a compound designated as an intercalationcompound and subsequently expanded in plasma; and using the expandedgraphite as one of an adsorption material, a sealing material, athermally conductive material and a thermally insulating material in aform selected from the group consisting of a loose material, films,plates and shaped bodies.
 39. The method according to claim 38, whichfurther comprises using the expanded graphite as a latent heat store bymixing or impregnation of a graphitic or partially graphitic materialwhich has an average particle size in the range from 10 nm to 10 mm andhas been treated with plasma in a pressure range from 5,000 Pa to300,000 Pa with a phase transition material selected from the groupconsisting of paraffins, sugar alcohols, gas hydrates, water, aqueoussolutions of salts, salt hydrates, mixtures of salt hydrates, salts,eutectic mixtures of salts, alkali metal hydroxides, mixtures of aplurality of the abovementioned phase change materials, mixtures ofsalts and alkali metal hydroxides, and mixtures of paraffins and salthydrates.
 40. A method of using a graphite, which comprises the stepsof: providing an expanded graphite formed from a starting materialselected from the group consisting of natural graphite, compressedexpanded graphite, partially oxidized graphite, and graphite fibershaving a BET surface area of >30 m²/g and one of substances capable ofintercalation and mixtures of substances capable of intercalationreacting with the starting material resulting in a compound designatedas an intercalation compound and subsequently expanded in plasma; andusing the expanded graphite for a formation of composites with organicmaterials.
 41. The method according to claim 40, which further comprisesproviding polymers as the organic materials.
 42. A method of using agraphite, which comprises the steps of: providing an expanded graphiteformed from a starting material selected from the group consisting ofnatural graphite, compressed expanded graphite, partially oxidizedgraphite, and graphite fibers having a BET surface area of >30 m²/g andone of substances capable of intercalation and mixtures of substancescapable of intercalation reacting with the starting material resultingin a compound designated as an intercalation compound and subsequentlyexpanded in plasma; and using the expanded graphite for formingcomposites with inorganic materials.
 43. The method according to claim42, which further comprises providing mineral building materials as theinorganic materials.